Fort Lauderdale, Fla. — Some of NASA’s most accomplished astronauts, including four of the 12 who’ve walked on the moon, are accusing President Obama of reducing America’s space program “to mediocrity” by scrapping plans for further lunar exploration.

In a searing open letter coinciding with Mr. Obama’s visit to Florida’s Kennedy Space Center this week, veterans of the Apollo, Mercury, Gemini, and space shuttle programs say the president is “throwing away” America’s dominance in human spaceflight after “50 years of unparalleled achievement.”

“We strongly urge you to drop this misguided proposal that forces NASA out of human space operations for the foreseeable future,” they wrote in the letter, released two days before Obama will visit Cape Canaveral to explain his decision to shut down the next-generation Constellation program, which was to have taken astronauts back to the moon by 2020.

“For those of us who have accepted the risk and dedicated a portion of our lives to the exploration of outer space, this is a terrible decision,” the letter says.

The letter comes during commemorations this week for the 40th anniversary of one of NASA’s finest achievements – bringing the Apollo 13 astronauts home safely after an onboard explosion. Messrs. Haise and Lovell, the two members of the crew still alive, attended events at the Kennedy Space Center and at the Adler Planetarium in Chicago.

Obama announced earlier this year that he planned to ax for budgetary reasons the Constellation program, first outlined in predecessor George W. Bush’s Vision for Space Exploration in 2004. Development of the new Orion spacecraft and the Ares rockets that were to lift them into orbit had already cost more than $9 billion over the previous five years.

While proposing to give NASA $100 billion over the next half decade, Obama pledged extra funds for scientific research and support of commercial enterprises that might eventually take American astronauts into space.

Obama’s plan would have NASA focus on developing technologies for powerful rockets that could take astronauts beyond Earth orbit, although the timetable and destinations are more open-ended.

But the letter’s signatories say this is not the right time for Obama’s proposals. The space shuttle program is slated is to end later this year, which raises the prospect of the United States relying on Russia to ferry its astronauts to the International Space Station.

“We are very concerned about America ceding its hard earned global leadership in space technology to other nations,” the letter says. “We are stunned that, in a time of economic crisis, this move will force as many as 30,000 irreplaceable engineers and managers out of the space industry.”

Thousands of space shuttle engineers and other agency workers staged a rally last weekend outside the space center to protest against the job cuts.

Senators representing Florida and Texas, where most of NASA’s workers are employed, have promised to fight Obama’s proposals in Congress.

Not all of NASA's noted alumni are critical of the Obama’s plans, however. In February, Buzz Aldrin, the second man to step on the moon, issued a statement applauding Obama's vision, saying: "A near-term focus on lowering the cost of access to space and on developing key, cutting-edge technologies to take us further, faster, is just what our Nation needs to maintain its position as the leader in space exploration for the rest of this century."

Still, those who signed the letter worry that a vital opportunity to educate and inspire American youths is being lost.

“We see our human exploration program, one of the most inspirational tools to promote science, technology, engineering and math to our young people, being reduced to mediocrity,” they wrote. “This is not the time to abandon the promise of the space frontier for a lack of will or an unwillingness to pay the price.”